There are many different kinds of canoes each with their own shapes and uses. A flat bottom canoe has wonderful primary stability (hard to get it to begin to tip over) but lousy secondary stability (once it tips a bit, watch out, you're going in). V shaped hulls have less primary stability but they have better secondary stability.

Pontoons are used when you need to use the wrong kind of canoe for your purpose - like standing up fishing in a v hulled canoe.

This product is already out there.

These have existed for hundreds of years at least. It's called an outrigger canoe. I'm pretty sure they were invented by the Polynesians. The outrigger is the arm that you speak of. It provides floation so you can't tip against the direction that it's tied to and the weight of the outrigger also serves to balance out tipping on the other side. These don't create too much drag either.

Instablity is a feature, not a bug.

It aids manouverability.

Ian Dickson

As we would say in the bakery, 'Baked.'

For lakes, bays, big water, etc. a stabilizer provides peace of mind and added security. Maneuverability is secondary to safety for small craft on big water. Especially if the weather decides to take an abrupt turn for the worse. Canoe stabilizers (canoe outriggers) exist in a variety of forms.

http://www.sailboatstogo.com/images/floats_ethafoam_on_Canoe.jpghttp://www.w2d.com/mermaid/sail400.jpg

IMHO, they're a stupendous bit of equipment allowing you to use a canoe where it normally wouldn't be such a good idea, such as in the instance of high wind, wake, or in conjunction with a motor or sail. Can't see much utility in them for smaller rivers or stream floating, where maneuverability is the key to not getting swamped.